Preparation of chemicals for kraft pulping and bleaching and apparatus therefor



y 5, 1969 G o. WESTERLUND 3,44 ,778

PREPARATION OF CHEMICALS FOR KRAFT PULPING AND BLEACHING AND APPARATUS THEREFOR Filed June 20, 1966 Sheet of 7 MERCURY CELL POLYSULPHIDE REACTOR ELEC'IROLYS IS I 17 NaClO 11 1a 19 2o HYDROGEN CHLORIDE COMBUSTION CHLORATE GENERATOR 22 CHLORIDE DIOXIDE 25 GENERATOR FIG! May 6, 1969 G. o. WESTERLUND' 3,442,778

EACHING AND PREPARATION OF CHEMICALS FOR KRAFT PULPING AND EL Sheet APPARAIUS THEREFOR Filed June 20, 1966 mwmwmbm NAN 25mm OHN mzwmm y 6, 1969 G o. WESTERLUND 3,442,778-

PREPARATION OF CHEMICALS FOR KRAFT IULIING AND BLEACHING AND APPARATUS THEREFOR Sheet Filed June 20, 1966 now now mow -3 n mum mow M I x vow h wow how y 1969 G. o. WESTERLUND 3,442,778

PREPARATION OF CHEMICALS FOR KRAFT PULPING AND BLEACHING AND APPARATUS THEREFOR Filed June 20. 1966 Sheet 4 of 7 3,442,778 CHING AND Sheet 5 of 7 APPARATUS THEREFOR EON mESQFSwG ma o;

May 6, 1969 e o. WESTERLUND PREPARATION OF CHEMICALS FOR KRAFT PULPING AND, BLEA Filed June 20, 1966 y 1969 e. o. WESTERLUND 3,442,778

PREPARATION OF CHEMICALS FOR KRAFT PULPING AND BLEACHING AND APPARATUS THEREFOR Filed June 20, 1966 Sheet 6 of 7 FINES 7 12 D ISSOLVER y 1969 cs. 0. WESTERLUND 3,442,778

PREPARATION OF CHEMICALS FOR KRAFT PULPING AND BLEACHING AND APPARATUS THEREFOR Filed June- 20, 1966 Sheet 7 of '7 CAUSTIC 9 1 4 STORAGE United States Patent 3,442,778 PREPARATION OF CHEMICALS FOR KRAFT PULPING AND BLEACHING AND APPARA- TUS THEREFOR Giithe 0. Westerlund, Vancouver, British Columbia, Canada, assignor to 'Chemech Engineering Ltd., Vancouver, British Columbia, Canada Filed June 20, 1966, Ser. No. 558,753 Int. Cl. B01k 3/00; C01b 7/00, 17/22 US. Cl. 20495 19 Claims This invention relates to a process for the preparation of chemicals, in balanced amounts for kraft pulping and bleaching. More particularly, it uses common chemicals, i.e., salt, sulphur and water, and electrical power, to provide such chemicals.

Heretofore, pulping and bleaching chemicals which were required in the kraft pulp industry were obtained from chemical producers. For this reason, fully bleached kraft pulp mills, long distances from chemical suppliers were uneconomical.

In certain countries of the world, particularly in North America and Scandinavia the trend is towards small captive chemical plants integrated with the pulp mill on the same site. This trend is gaining momentum for the following reasons:

(1) The competitive nature of the pulp market makes it necessary to find new methods for lowering the cost of pulp. Chemical costs may be as high as 25% of the manufacturing costs.

(2) Chemical manufacturing technology is today available to pulp mills.

(3) The change in bleaching chemicals, i.e., chlorine dioxide has largely replaced hypochlorite and can substitute a large part of the chlorine now that the cost has been further reduced by improving the manufacturing process.

(4) Transportation costs are minimized by transporting a few low cost raw materials, rather than the more expensive and dangerous chemicals from chemical suppliers.

(5) The size of pulp mills has increased and thus also the requirements for chemicals.

A broad object of the present invention is economically to produce all of the basic chemicals required by the kraft pulp mill as well as balancing the output of these chemicals with the requirement and using a minimum number of raw materials.

By another aspect of this invention there is provided a process in which electric power, common salt, sulphur and water are the only raw materials.

By a broad aspect of the present invention, a process is provided for the preparation of chlorine dioxide, chlorine, hydrogen, hydroxide selected from the group consisting of alkali metals and alkaline earth metals and sulphide selected from the group consisting of alkali metals and alkaline earth metals for use in kraft pulp mill operations from chloride selected from the group consisting of alkali metals and alkaline earth metals, sulphur and water, which process comprises the following interrelated combination of steps.

In one preferred aspect, step (A) involves a mercury cell electrolysis procedure.

In one embodiment of the aforesaid preferred aspect, the amalgam of the alkali metal or alkaline earth metal with mercury is fed as one of the reactants for step (G) and for step (H).

In another embodiment of the aforesaid preferred aspect, the mercury formed in steps (G) and (H) is fed as feed for step (B).

In another preferred aspect, step (B) involves an electrolysis procedure.

3,442,778 Patented May 6, 1969 ice 'In yet another preferred aspect the alkali metal or alkaline earth metal chloride formed in step (D) is recycled to the feed for steps (A) or (C).

In still another preferred aspect, the chlorine gas produced in step (E) is recycled as feed for step (C).

In a still further preferred aspect, step (E) also produces excess alkali metal or alkaline earth metal ions which are recycled to the feed for steps (A) or (C).

By a still further preferred aspect, step (A) comprises:

(i) Dissolving the chloride selected from the group consisting of alkali metals and alkaline earth metals in the Water to form chloride brine selected from the group consisting of alkali metals and alkaline earth metals;

(ii) Electrolyzing a portion of said brine in a mercury cell whereby to produce gaseous chlorine at the anode, a portion of which is recovered and amalgam selected from the group consisting of alkali metals and alkaline earth metals at the cathode.

By one embodiment of this preferred aspect, step (G) comprises:

(i) Reacting a portion of the amalgam selected from the group consisting of alkali metals and alkaline earth metals from (ii) with polysulphide selected from the group consisting of alkali metals and alkaline earth metals whereby to form mercury for (ii) and to form sulphide selected from the group consisting of alkali metals and alkaline earth metals a portion of which is recovered;

(ii) Reacting a portion of said sulphide selected from the group consisting of alkali metals and alkaline earth metals from (i) with sulphur whereby to form polysulphide selected from the group consisting of alkali metals and alkaline earth metals for (i).

By a second embodiment of this preferred aspect, step (H) comprises:

(i) Decomposing a portion of amalgam selected from the group consisting of alkali metals and alkaline earth metals from (ii) whereby to form hydroxide solution selected from the group consisting of alkali metals and alkaline earth metals, a portion of which is recoverd; gaseous hydrogen, a portion of which is recovered; and mercury for (ii).

By a further aspect of this invention, step (B) comprises:

(i) Electrolyzing a mixture of brine of (i) with brine formed by (iii) whereby to form chlorate selected from the group consisting of alkali metals and alkaline earth metals and to form gaseous hydrogen, a portion of which is recovered;

(ii) Reacting a portion of said gaseous chlorine from (ii) with a portion of said gaseoushydrogen from (i) to form hydrogen chloride in gaseous or aqueous solution form;

(iii) Reacting a portion of said chlorate selected from the group consisting of alkali metals and alkaline earth metals from (i) with a portion of said hydrogen chloride from (ii) whereby to form chloride selected from the group consisting of alkali metals and alkaline earth metals for (i) and to form gaseous chlorine for (ii) and water for electrolysis for (i), and to form and recover chlorine dioxide.

By a preferred embodiment of any of the above described aspects of this invention the starting materials are sodium chloride, sulphur and water and wherein the final products are chlorine dioxide, chlorine, hydrogen, sodium hydroxide and sodium sulphide.

By yet another aspect of this invention apparatus is also provided for the preparation of chlorine dioxide, chlorine, hydrogen, sodium hydroxide and sodium sulphide for use in kraft pulp mill operations from sodium chloride, sulphur and water which process comprises the following interconnected series of units:

(A) (i) A mercury cell operative to produce gaseous chlorine at the anode and sodium amalgam at the cathode;

(ii) Means at the anodic outlet of (A) (i) for recovering a portion of the chlorine produced at the anode;

(B)(i) An electrolytic cell operative to electrolyze brine to form sodium chlorate and gaseous hydrogen;

(ii) Means connected to electrolytic cell (B)(i) for the recovery of a portion of the gaseous hydrogen so formed;

(C) (i) A hydrogen chloride combustion chamber operative to produce hydrogen chloride;

(ii) Means interconnecting the inlet of said combustion chamber (C) (i) with a gaseous hydrogen outlet from said electrolytic cell (B)(i) and for interconnecting the inlet of said combustion chamber (C)(i) with a gaseous chlorine outlet from mercury cell (A) (i); and

(iii) Means for recovery of a portion of said hydrogen from electrolytic cell (C) (i);

(D)(i) A chlorate electrolyzer and generator operative to produce sodium chlorate and hydrogen;

(E) (i) A chlorine dioxide generator operative to form gaseous chlorine, water, sodium chloride, and chlorine dioxide;

(ii) Means connecting the inlet of said chlorine dioxide generator (E) (i) with the chlorate product effluent line of chlorate generator (D)(i) and means connecting the outlet of said chlorine dioxide generator (E)(i) with the sodium chlorate inlet line of electrolytic cell (B)(i);

(iii) Outlet means from chlorine dioxide generator (E) (i) to recover said chlorine dioxide; and

(iv) Means connecting a chlorine outlet from generator (E) (i) with the inlet to combustion chamber (C) (i);

(F)(i) A sulphide reactor operative to form mercury and sodium sulphide;

(ii) Means connecting the mercury outlet line of said sulphide reactor (C)(i) with the mercury inlet line of mercury cell (A) (i) and/or to amalgam decomposer (iii)) Means connecting the inlet of said reactor (C) (i) with the amalgam outlet line of said mercury cell (A) (i); and

(iv) Means for the recovery of a portion of said sodium sulphide of (C)(i);

(G)(i) A polysulphide reactor operative to form sodium polysulphide;

(ii) Inlet means for the introduction of sulphur to said reactor (G)(i);

(iii) Means connecting the sodium sulphide outlet of said sulphide reactor (C) (i) with the inlet of said reactor (G)(i);

(iv) Means connecting the polysulphide outlet of said reactor (G) (i) with the inlet of said reactor (C) (i); and

(H)(i) A decomposer to form sodium hydroxide solution, gaseous hydrogen and mercury;

(ii) Means connecting the amalgam outlet of mercury cell (A)(i) with the inlet of said decomposer (H)(i) and/or means for connecting excess amalgam outlet means from sulphide reactor (F)(i);

(iii) Inlet means for water to said decomposer (iv) Means for the recovery of product hydroxide from decomposer (H)(i);

(v) Means for the recovery of product hydrogen from decomposer (H)(i); and

(vi) Means connecting the mercury outlet of decomposer (H) (i) with the mercury inlet of mercury cell The preferred chemical plant for carrying out the process of the present invention consists of suitable apparatus for the electrolysis of brine to produce chlorine, sodium amalgam and sodium chlorate; apparatus for the treatment of the sodium amalgam to produce sodium hydroxide and sodium sulphide; and apparatus for the treatment of sodium chlorate to produce chlorine dioxide.

In the accompanying drawings:

FIGURE 1 is a schematic representation of the entire process of one preferred aspect of the present invention,

FIGURE 2 is a schematic flow sheet of a typical amalgam cell process for the production of chlorine, which may be used in carrying out one step of the preferred process of the present invention,

FIGURE 3 is a diagrammatic horizontal sectional view of a typical amalgam cell which may be used in the present invention, the cell being that known as an I. G. Farben horizontal cell,

FIGURE 4 is a partial cross-section of the cell of FIG- URE 3,

FIGURE 5 is a schematic diagrammatic representation of a process for carrying out another step in the present invention, i.e., chlorine dioxide manufacture,

FIGURE 6 is an idealized schematic representation of the chemical equations of the process represented in FIG- URE 5,

FIGURE 7 is a schematic fiow sheet for carrying out another step of the process of the present invention, i.e., sodium sulphide manufacture showing Mathieson-type reactor, and

FIGURE 8 is a schematic horizontal sectional view of a compartment reactor for sulphide, used in the process shown in FIGURE 7.

Turning first to FIGURE 1, it is seen that sodium chloride 10 and water 11 are used to form a sodium chloride brine 12. The brine 12 is subjected to mercury cell electrolysis in a mercury cell 13, and the products of this mercury cell electrolysis are chlorine 14 (which is reused) chlorine 15 (which is recovered) and a sodium amalgam 16 (which is reused).

In start-up, a portion of the sodium chloride brine 12 is subjected to a chlorate electrolysis in chlorate electrolysis cell 17. Thereafter, the process of the present invention is self-sufficient with respect to electrolyte because of the return of the effluent liquor 23 and 27 from the chlorine dioxide generator. The product from cell 17 and from the chlorate generator part of the present system is sodium chlorate 18 (which is reused), hydrogen gas 19 (which is reused) and hydrogen gas 20 (which is recovered).

Hydrogen gas '19 and chlorine gas 14 are reacted in hydrogen chloride combustion chamber 21 and the gaseous or liquid hydrogen chloride 22 so formed is conducted to a chlorine dioxide generator 25, Where it is reacted with sodium chlorate 18. The products of this reaction are sodium chloride 23 (which is recycled to electrolyser 17), water 27 (which is recycled to electrolyser 17), chlorine dioxide 26 (which is recovered), and chlorine gas 28 (which is used as part of the make-up reactant in hydrogen chloride combustion chamber 21).

A stoichiometrically balanced portion of sodium amalgam 16 is fed to sulphide reactor 29, and the remainder is fed to electrolytic decomposer 30. Alternatively, all the amalgam may be fed to the sulphide reactor, a portion of which is being used for reaction with polysulphide. The excess amalgam and released mercury may then be transferred to the decomposer 30. In the sulphide reactor 29, sodium polysulphide 30 is reacted with the sodium amalgam 16 to form mercury 31 (which is recycled to the mercury cell 13), sodium sulphide 32 (which is recovered) and sodium sulphide 33. Sodium sulphide 33 is fed to a polysulphide reactor 34, where it is reacted with sulphur 35 to form sodium polysulphide 30.

In electrolytic decomposer 30, sodium amalgam 16 is reacted with water 36 to form sodium hydroxide (which is recovered), hydrogen gas 38 (which is recovered) and mercury 39 (which is recycled to the mercury cell 13).

Thus, by the process of the present invention, useful kraft pulp mill chemicals, i.e., gaseous chlorine, chlorine dioxide, sodium sulphide and sodium hydroxide, as well as gaseous hydrogen (which may be burned to generate electric power or otherwise utilized) may be conveniently (A) The mercury cell process Numerous cell designs are available, but a typical one is shown in FIGURES 3 and 4 which will 'be described later. In essence, salt is dissolved in water to make brine which is fed to the electrolyser for production of chlorine and sodium amalgam.

water NaCl NaCl... r Na+ G1- Anode reaction: 01- to 01 e- (II) Cathode reaction: Na+ e- Hg Na-Hg (III) The simplified procedure described above will now be.

described with particular reference to FIGURE 2. Further technical information may be obtained from the publications Chemical Engineering Progress, September 1950, pages 440-445 and Modern Chemical Processes, vol. III, pages 237-248.

The rock salt is fed by rock salt elevator 201 to a salt dissolving tank 202. The raw brine is then pumped via line 203 and pump 204 to a raw brine storage tank 205.

Brine from the brine storage tank 205 is pumped via pump 207 and line 208 to a brine heater 208 and thence to a treating settler 209. At the settler, the pH of the brine is raised to 10.0 by the addition of sodium hydroxide and a coagulant such as sodium carbonate. This caustic treatment precipitates out iron, magnesium, and other impurities, which would otherwise cause excessive hydrogen formation in the electrolyser. The bottom material from the settler, consisting mainly of ferric hydroxide, calcium and magnesium hydroxide or carbonates, is discarded via sludge line 21 1.

All equipment up to the filters is carbon steel or cast iron. As protection against brine contamination by heavy metals, all equipment including the filter and beyond is rubber-lined.

The overflow from treater 209 is led via a surge tank 212 and pumped by pump 213 and line 214 to a filter 215, and then via line 216 to purified brine storage tank 217. The brine from the purified brine storage tank 217 is pumped via pump 218 and line 219 to a brine head tank 220. If the brine is not at temperature of 30-50" C., it should pass through a heat exchanger (not shown) where its temperatures is raised from about 30 to 50 C. by hot weak brine flowing through the tubes. The brine temperature is increased to reduce the required cell voltage and hence the power consumption during the subsequent electrolysis.

After passing through the heat exchanger, the brine is continuously pumped to the brine head tank 220. The pH in this tank should be controlled to 2.5 by the addition of hydrochloric acid 221. This acid pH eliminates the need for freeing the brine of calcium sulfate.

At a temperature of about 50 C. the brine flows by gravity to the electrolytic cells only one of which 222 is shown. The flow rate to the individual cells is properly adjusted so that the outlet brine from the cells contains a preselected amount of sodium chloride and the temperature does not exceed 85 C. The outlet brine concentration could actually be reduced still further, but this introduces the problems of increased cell voltage and increased concentrations of hydrogen in the chlorine gas. Furthermore, the existing operating level permits a rapid flow of brine to be maintained in the electrolyzer. Otherwise, the temperature of the cell would increase excessively, causing the rubber lining to undergo accelerated attack by chlorine.

Rubber-lined diaphragm control valves are located at each cell, and the brine is distributed to the cells in rubber-lined headers. Glass headers might also be used, although rubber-lined equipment is generally preferred for its mechanical ruggedness.

In the electrolyzer compartment of the mercury cell 222, chlorine gas 223 and sodium amalgam 224 are produced electrolytically. In each cell, a plurality of graphite electrodes 225 are used as the anodes, while the pool of mercury 226 serves as the cathode. The chlorine gas is drawn ofi in stoneware mains under a slight suction of 0.25 inch water. At the same time, the sodium combines with the mercury 227 to form an amalgam containing about 0.15% sodium. The reactions involved are:

Anode: C1-- e+ /2 C1 Cathode: Na+ +e- Na Na-i-xHge NaHg The amalgam 224 flowing along the sloping bottom of the electrolyzer, passes through a seal before it conveyed to the sulphide reactor 29 or the electrolytic decomposer 30, where operations will be described in greater detail hereinafter.

The cells, electrically in series, are equipped with cutout switches to permit each cell to be short-circuited when necessary. Copper or aluminum bus bars run from cell to cell, with one bus bar for each pair of anodes. Ordinarily, the anodes can 'be used continuously for about a year.

As the depleted brine 228 saturated with chlorine 223, leaves the cells 222 at about 82 C., it passes through porcelain pipe to an outlet bn'ne receiver 229 lined with rubber and acidproof brick. To this receiver, 30% hydrochloric acid 230 is added automatically to maintain a pH of 2.2. At this pH, the hypochlorous acid and chlorate present in the brine are converted to chlorine and Water.

Next, the brine is pumped through the declorination system 231, which, by removing almost all the chlorine from the brine, not only eliminates a possible health and odor problem but also minimizes the corrosion of the brine processing equipment, particularly the brine well piping. The first unit of the declorination system is a rubber-lined flash tank 232, evacuated to a pressure of about 13 inches of mercury by stoneware vacuum pumps 233. The recovered chlorine, after compression via pump, goes to the chlorine discharge line 234, from the electrolyzer. The brine, partially free of chlorine, flows down a barometric leg 235 which eliminates the need for pumping the brine out of the vacuum-operated flash tank.

In the next step, the brine goes to a degassing tower 23th In this tower, which is half filled with brine, the brine is subjected to a rapid flow of compressed air 237. After passing through the tower 236, part of the weak brine is sent through the brine heat exchanger (previously referred to as an optional feature), while the remainder by-passes this unit. Ordinarily, the necessary amount of heat is transferred to the strong brine flowing in the outer jacket when only about half of the weak brine passes through this heat exchanger. To all the brine, caustic 238 is added to adjust the pH to 10. Because of this alkaline pH, carbon steel or cast iron can be used in the subsequent brine equipment. The added caustic also precipitates out some magnesium in the brine well. In addition sulphur dioxide 239 is added before the brine is stored in brine storage tank 240, from which it is pumped via line 241 and pump 242 to the salt dissolving tank 202.

The chlorine in line 234 is pumped through a chlorine cooler 243 and fed to the bottom of a countercurrent drying tower 244. Here, it is dried by contact with 98% sulphuric acid 245. The chlorine is then pumped via compressor 246 and filter 247 to a dry chlorine gas storage at 40 p.s.i.g.

This represents chlorine gas 15 in FIGURE 1. A portion of this chlorine gas may be used as chlorine gas 14 later in the process.

Turning now to FIGURE 3 it is noted that the cell 7 includes a sloping pool of mercury 226 as the cathode and a plurality of graphite anodes 225. The mercury 227 is circulated by means of pump 301 and the mercury amalgam 224 is removed from the cell at outlet 302. The brine is admitted via inlet 303 and the chlorine and brine are removed via outlet line 304.

Turning now to FIGURE 4, it is seen that the bottom of the electrolyzer consists of a steel channel 401. The sides of the electrolyzer are channels 402 covered with hard rubber 403 on the upper flange and web. The ends are closed by inlet and outlet boxes (not shown), which are cast iron, lined with hard rubber. The electrolyzer cover 404 is a single piece of reinforced steel, rubbercovered on all surfaces exposed to chlorine. The cover rests on a soft rubber gasket 405 with C-clamps 407 used to provide a gas-tight seal.

The graphite anodes 225 are supported from the cover. Lead-in posts 406 pass through the cover and are sealed with flexible rubber. Two metallic lead-ins 408 protected with a porcelain sleeve, are used for each anode block. To promote the flow of chlorine to the upper surface of the anode, slots are cut into the graphite, and several holes are drilled above each slot (not shown).

From the above description, it is seen that in the amalgam process for the production of chlorine an aqueous solution of a chloride salt is electrolyzed. Chlorine gas is evolved at graphite anodes, and the metal of the salt in question is deposited on a flowing mercury cathode. The free metal dissolves in the mercury to form an amalgam, which is withdrawn from the electrolysis compartment. In most existing plants, the sodium amalgam is decomposed with water to form caustic soda and hydrogen gas and this procedure will be described in greater detail hereinafter. The caustic so produced is exceptionally pure and can be made in strengths up to 70% NaOH. After stripping the sodium from the amalgam as will be described hereinafter the mercury is returned to the electrolysis cell as will be described hereinafter.

The sodium-type amalgams, i.e, those of sodium, potassium, rubidium, cesium, calcium, strontium and barium are of a special type having the following characteristics:

(a) They react completely rather than superficially,

(b) They have a low solubility in mercury,

(c) They possess undervoltage with respect to pure metal,

((1) They form compounds with mercury, and

(e) They can be formed by aqueous electrolysis.

For sodium, potassium, lithium and calcium, which belong to the sodium-type, the solution pressure of the amalgam is considerably less than that of the free metal. One may say, therefore, that the alkali and alkaline earth metals are made more noble by amalgamation.

This property of undervoltage of the alkali metal amalgams is important. For many reactions, use of the free alkali metal would be technically difficult, or impossible, to say nothing of the great expense. Sodium metal, for example, reacts violently with water, but sodium amalgam reacts hardly at all, excepting under special and controlled conditions. The overvoltage of hydrogen gas against amalgam is high. Thus it is possible to bring about a controlled reaction between sodium amalgam and various other reactive compounds in aqueous solution, with high efficiency and little loss due to attack of the sodium on the water to form hydrogen.

Chlorine dioxide process Apparatus for this system, including electrolytic chlorate manufacture, has been developed by the present invention, and is disclosed and claimed in Canadian application Ser. No. 906,199.

The process is self-contained regarding electrolyte for production of chlorate after the initial charge, and is based on following main reactions.

Chlorate electrolysis:

2NaCl+6H O+ l2 Faradays 2NaClO 6H (IV) Combustion:

Chlorine dioxide generation:

2NaClO -l-2HCl 2NaCl+2HClO (VI) 2HC'1O +2'HCl 2ClO +Cl +2H O (VII) When balanced, the overall reactions may be summarized as follows:

4H O+Cl +l2 Faradays=2ClO +4H (VIII) Turning first to FIGURE 5, an electrolytic cell 510, which may be that disclosed and claimed in pending Canadian application Ser. No. 901,153, filed Apr. 24, 1964, 0perates to electrolyze an aqueous solution of a metal chloride, for example sodium chloride, and the efiiuent leaves via line 511, from whence the liquid products proceed via line 512 to reacting chamber 513, and the gaseous products proceed via line 514 to a combustion chamber 515, whose purpose and function will be described hereinafter. The liquid is induced to react in the reaction chamber 513 and is recycled via pump 516 and line 517 through cooler 518 and line 519 to electrolytic cell 510.

A branch line 520 cycles part of the reaction products formed in the reacting chamber 513 through filter 521 through means of pump 522 and line 523. Filter liquid 515 flow is by means of lines 525 and 526 to the bottom of a generator 527. A gas separation zone 528 separates gaseous product in generator 527 from liquid products, and the liquid products are recycled by means of pump 524 and line 529 to line 526 for reaction again in the generator. A portion of the liquid product is led via line 530 to the intake side of pump 516. From there, they may be recycled to cooler 518 and thence to electrolytic cell 510 for reconcentration, to filter 521 and thence to generator 527. The off-gases from generator 527 are fed via line 531 to the bottom of absorber 532.

The off-gases from electrolytic cell 510 pass via line 514 to combustion chamber 515 as stated hereinabove, where they are reacted with chlorine gas admitted via line 532. Hot gases from combustion chamber 515 are compressed via pump 533, pressure regulated through recirculation line 534 and fed via line 535 to the bottom of generator 527. Alternatively, the compression may be done ahead of the combustion chamber 515 which simplifies material specification for pump since the temperature is relatively low before the combustion chamber. A branch line 536 off line 535 leads to a water scrubber 537, where excess gases are scrubbed with water admitted through line 538. The effluent from scrubber 537 is passed via line 539 to the bottom of generator 527. Off-gases from scrubber 537 are vented via line 540.

As stated hereinbefore, exit gases from generator 527 are passed via line 531 to the bottom to absorber 532. Here they are absorbed by water admitted via line 541. Desired chlorine dioxide is recovered via line 542, while off-gases are passed via line 543 to line 514 and back to the combustion chamber 515.

Turning now to FIGURE 6, there is shown in idealized form, interconnected electrolysis zone 550, generator zone 551, combustion zone 552, absorption zone 553 and scrubbing zone 554. The electrolysis zone 550 where the reaction 2[NaCl+3H O+6 Faradays- NaClO +3H (where the metal is sodium) takes place, may be considered to be coextensive with electrolytic cell 510, reacting 9 chamber 513, coo-ling 518, and the associated interconnections shown in FIGURE 5.

The generator zone 551, where the reactions (where the metal is sodium) takes place may be considered to be coextensive with the generator 527, the filter 521 and the associated interconnections shown in FIG- URE 5. One of the reactants, i.e. aqueous sodium chlorate, for the reaction in the generation zone 551, is fed to the generation zone 551 from the electrolysis zone 550 via line 526 (as stated with reference to FIGURE in the form of an aqueous solution (i.e., NaClO +nH O). The other reactant, hydrogen chloride gas, for the reaction in the generation zone 551 is fed to the generation zone 551 via line 535 (as also shown in FIGURE 5) in the form of impure water wet gas (i.e., 4HCl+H O+H The liquid efiiuent (i.e., NaClO +NaCl+H O) from generation zone 551 passes via line 530 (as shown in FIGURE 5) to be recycled to the electrolysis zone 550. Fresh water, with any additional reagents may also be added via line 544. The gaseous eflluent (i.e.,

from generation zone 551 passes via line 531 (as also shown in FIGURE 5) to the absorption zone 553. A recirculation line 529 assures greater reaction volume in the generation zone and hence increases the efiiciency of the process.

As stated hereinabove, the liquid eflluent (i.e.,

.passes from the electrolysis zone 550 to the generator zone 551 via line 526. The gaseous efiluent (i.e. predominantly 3H is conducted via line 514 (as also shown in FIGURE 5) to combustion zone 552.

Combustion zone 552 may be considered as being coextensive with combustion chamber 515, pump 535 and the associated interconnections shown in FIGURE 5. In the combustion zone the following reactions take place H /2O H O (to use up incidentally present oxygen) and H +Cl 2HCl Since the origin of the hydrogen and oxygen in the above equations has been explained, suffice to say that the chlorine for the reaction in the combustion zone 552 has two derivations: as freshly added chlorine gas via line 532 (as also shown in FIGURE 1) and as generated chlorine from the absorption zone 553 via line 543 (as also is shown in FIGURE 5). The sole effluent from the combustion zone, namely the hot or cooled exhaust gases (i.e., 4HCl+H O+H a small amount of CO from the electrolysis by-reaction) is fed primarily via line 535 (as also shown in FIG. 5) to the generation zone 551. A branch line 536 (as also shown in FIG. 5) conducts a bleed-01f amount of such gaseous efiiuent to a scrubbing zone 554.

Absorption zone 553 (equivalent to absorber 532) is fed with efiluent (namely 2ClO +Cl +2H O and recycled dilution gases which are mainly hydrogen but also include a small amount of carbon dioxide) from generator zone 551 via line 531 (as' also shown in FIGURE 5). Here the gases are contacted with water, admitted via line 541 (as also shown in FIGURE 5) and the water soluble chlorine dioxide emerges as an aqueous solution via line 542 (as also shown in FIGURE 5) (i.e., as 2ClO +nH O). The unabsorbed gases, C1 and recycled gases are led via line 543 (as also shown in FIG. 5) to combustion zone 552.

The scrubber zone 554 (equivalent to scrubber' 537) is provided to recover excess hydrogen chloride. The stream of gases (4HCl-l-H 0-l-H from combustion zone 552 led -via line 536 to scrubbing zone 554 is contacted with water in that zone, admitted via line 538 (as also shown in FIGURE 5). Liquid effluent hydrochloric acid leaves via line 555 to a hydrochloric acid storage tank 556 or is fed to the chlorine dioxide generator 551. If more concentrated hydrochloric acid is required, the dilute acid may be recycled via line 557.

The process of this step of the present invention may thus be summarized in the following terms:

Chlorine (which may be chlorine gas 14 and/or chlorine gas 28) is fed by a line 532 to the chamber 515 where the gas is subjected to combustion with the gaseous mixture (which may be hydrogen gas 19) from another feed line 514. The hot gases from combustion chamber 515 are compressed (533) and pressure regulated by means of a recirculating line 534. The compressed gas is fed to generator 527 by a line 535 which has a branch line 536 for excess gases. These exit gases are scrubbed by scrubber 537 for hydrogen chloride. The overflow hydro-' chloric acid is fed via line 539 to generator 527. Exit gases from scrubber 537 are mainly hydrogen. The amount of water fed to scrubber 537 via line 538 is regulated to maintain desired volume of liquor in process system.

Hydrogen chloride gas and hydrochloric acid react in generator 519 with chlorate to produce chlorine dioxide and chlorine gases. The gases are separated from liquor by providing sutficient surface area 528 and enter absorber 532 where chlorine dioxide is dissolved in water and leaves the system via line 542 as a diluted solution partly contaminated with chlorine. The exit gas from absorber 532 contains most of the C1 produced and the inert H which is used for agitation in the reactor and as a diluting agent of the C10 and C1 for the purpose of eliminating the explosion hazard from concentrated clO -gas. The gases are returned to chamber 515 for combustion, via line 543.

The effluent liquor from generator 519 is returned to the electrolytic cell system via line 530. In cell units 510, hypochlorite and hypochlorous acid are produced and liquor is transferred via line 512 to a reacting chamber 513 for formation of chlorate. A recirculating pump 16 forces liquor through a cooler 518, for control of cell liquor temperature, and back to the cell units 510.

The gases produced in cell unit 510, mainly hydrogen, are fed via line 514 to the chamber 515 for combustion. Generator reagent liquor from reacting chamber 513 is pumped (522) through a filter 521 for removing solid particles and fed to generator 527 or to generator recirculating line 52 9.

Sodium sulfide process Sodium amalgam from the mercury cell reacts with sodium polysulfide solution to form sodium sulfide and mercury:

A portion of the sodium sulfide solution is used for dissolving additional sulphur to make up the polysulfide solution for Reaction IX.

Reaction IX is carried out with excess of sodium amalgam which together with the released mercury is transferred to the decomposer for production of sodium Hydroxide.

The aforesaid process will now be described with reference to FIGURE 7. Further details of this procedure may be found in Chem. Engl. Progress, vol. 46, No. 9, 448-57, Robert B. MacMullin.

Sodium amalgam 701 (which is a stoichiometrically balanced amount of sodium amalgam 16) is pumped into sulphide reactor 29 at inlet 705 where it reacts with so- 11 dium polysulphide pumped from storage tank 702 via pump 703 and line 704 to inlet 705. An agitator 707 is provided in each cell compartment 708. The elfluent sodium sulphide is carried via line 709 past trap 710 to a fiaker 711. The overflow from the fiaker 711 is screened on screen 712 and the flake sodium sulphide 32 is formed (which is recovered). The fines 33 are passed to a fines dissolver 712 where it is mixed with water. The efiiuent is pumped via pump 713 and line 714 to a polysulphide reactor 34 where sulphur 35 is dissolved therein. Dissolver 35 is heated by a steam jacket 715. The sodium polysulphide is pumped via pump 716 and line 717 through filter 718 to the storage tank 702. Alternatively, sodium amalgam 701 may be fed to reactor 29 in excess of the stoichiometrically balanced amount and the excess and released mercury may be fed to the decomposer 901, FIGURE 9 for production of sodium hydroxide.

FIGURE 8 shows somewhat more clearly the reactor used in the process described in FIGURE 7. The reactor 29 may comprise a tubular shelf divided into a plurality (shown as compartments 708. Within each compartment 708 is an agitator 707. The polysulphide solution is pumped into the reactor 29 via inlet line 705 and the sodium sulphide is withdrawn via outlet line 709. The amalgam is pumped in via inlet line 706 and the produced mercury is withdrawn via outlet 720. It is noted that the fiow of polysulphide is concurrent with the amalgam.

The aforementioned process can be adapted to making crystalline Na S. For the latter, a polysulfide solution corresponding to the formula Na S is prepared, and the operating temperature is maintained high enough to prevent freeze-ups. Care must be taken not to strip completely the sodium from the amalgam, otherwise some mercury sulfide will form.

By compartmentizing, the free sulfur can be reduced to a low value. Sodium sulfide solutions containing 0.01% sulfur or less are practically water white, whereas the polysulfide solution fed to the cell is a deep red.

Because of solubility conditions it is impractical to make sodium sulfide in any strength above 60% Na S. For the production of sulphide of the highest purity and whitest color, it is essential to exclude air, which otherwise oxidizes the sulfide to thiosulfate, and to avoid contamination with such metals as iron, lead and zinc. The heavy metals in particular give the product a reddish cast.

The next stage of the process of the present invention is the decompsition of the sodium amalgam with water in the decomposer 30. This will be described in greater detail with respect to FIGURE 9.

The sodium amalgam is fed to decomposer 30 by means of amalgam pump 901. In the decomposer 30 the sodium reacts with deionized water 902 to form hydrogen gas 38 and caustic soda 37 solution. The reactions are:

The concentration of caustic soda produced, is controlled by the amount of water entering the bottom of the decomposer. The flow of deionized water to each decomposer is regulated by a needle valve and is measured with a rotameter. This purified water is obtained from a demineralizer employing anion and cation exchange resins. The treated water is stored in a rubber-lined head tank and is distributed in rubber-line pipe.

The hydrogen 38 and caustic 37 produced in the decomposer leave at the top. At the same time, a sump-type centrifugal pump continuously returns the stripped amalgam, containing less than 0.01% sodium to the inlet of the electrolyzer 13.

The hydrogen 38 as-it comes from the decomposers 30, is saturated with water and mercury vapor, and, in addition, contains some caustic mist. The gas is cooled in a water-jacketed heat exchanger to recover the mercury and caustic.

The impure caustic proceeds from caustic collection tank 903 via line 904 and pump 905 to settling tank 906. The sludge is discarded via line 907 and the effluent is pumped via pump 908 and line 909 through filter 910 to a surge tank 911. From here, it is pumped via pump 912 and line 913 to caustic storage tank 914.

In other words, caustic alkalis of commercial importance, i.e., the hydroxides of sodium, potassium and lithium can be made from the corresponding amalgam by decomposition with water. Most existing amalgam-type chlorine plants produce caustic soda; a few produce caustic potash; and so far as is known, there are no existing plants producing lithia.

Pure water in sufficient quantity is supplied to the decomposer to carry on the above reactions and to carry away the sodium hydroxide formed. The escape of hydrogen also evaporates water from the solution. Solutions containing up to NaOH can be produced directly in the decomposer.

Alkali metal amalgam reacts with water slowly because of the high over-voltage of hydrogen on the amalgam surface. The reaction can be promoted by placing a metal or graphite electrode in contact with the amalgam and the solution. This forms a voltaic battery. The driving force of the cell is the metal amalgam anode potential. Opposing it are the anode polarization, IR drop in the caustic solution, hydrogen over-voltage, hydrogen discharge potiential, and the IR drop of the metallic parts. The amalgam discharge potential depends solely upon the activity of the sodium in the amalgam and the activity of the sodium ion in solution. It is increased by (1) increasing the sodium concentration in the amalgam, (2) decreasing the sodium hydroxide concentration of the electrolyte, and (3) increasing the temperature. The rate of discharge depends upon the amalgam potential and also upon the area of the cathodic surface on which the hydrogen is deposited. In a given cell, therefore, the higher the ampere load, the lower the equilibrium strength of alkali that can be produced.

Conditiontions for producing sodium, potassium and lithium hydroxides differ, since they depend on such physical properties as the solubility of the metal in the amalgam; the solubility of the chloride in the brine used for electrolysis; solubility of the hydroxide in the water used for decomposition, and the various electrode potentials both in the electrlysis cell and in the decomposer. At the usual cell operating temperatures, sodium is the most soluble and lithium the least soluble in mercury; therefore the amalgam circulation/1000 amp. is necessarily greatest for lithium and least for sodium. Solubility of the chlorides is greatest for lithium and least for sodium; therefore the brine circulation must necessarily be greatest for sodium chloride. Solubility of the hydroxides is greatest for sodium and least for lithium; at 70, for example, solubility of NaOH is and it is practical to operate the decomposer at 70% strength. At the same temperature, solubility of KOH is only 60%, and it would be practical to operate the decomposer at a maximum of about 57%. At the same temperature, the solubility of LiOH is only 13%, and it will be impractical to make caustic lithia at any strength more than about 11%.

The two outstanding advantages of the mercury-cell process the production of caustic alkalies are: Commercial strength of caustic can be manufactured directly in the decomposer, and does not require further evaporation, and the caustic so produced is exceptionally pure.

The amalgam-cell process is equally adaptable for the production of high quality caustic soda and caustic potash. No changes in the plant are necessary in shifting from the production of one to the other. For the production of caustic lithia, some adaptations would have to be made.

Amalgam decomposers 30, can be classified as follows:

(A) The sloping, nearly horizontal trough in which the amalgam flows countercurrent to the water. The contact cathodes, are laid on the bottom of the trough so that they are in contact with both the amalgam and the water.

(B) The tower type which is filled with carefully sized granules of graphite, over which the amalgam and the water are caused to floW. Such towers may be operated either flooded or non-flooded with respect to the amalgam, and the fiow of water may be either parallel or countercurrent to the amalgam. A typical tower-type has been described in operation with respect to FIGURE 9. This is shown in greater detail, in diagrammatic form, in FIGURE 10.

The decomposer is a tubular member 915 fabricated from carbon steel and packed with lumps of crushed graphite 916. The packing is held between screens 917, compressed with jackscrews. A distributor plate 918 spreads the amalgam pumped in via inlet 919 evenly over the top of this packing. In the decomposer, the amalgam acts as the anode, which is short-circuited to the graphite, which functions as the cathode. The formation of this electric couple between the amalgam and graphite accelerates appreciably the conversion of metallic sodium to sodium ions in the presence of water. The water enters by inlet 920 and flows upwardly through the decomposer. Caustic produced this way is removed via caustic outlet 921. The gaseous hydrogen formed is removed through hydrogen outlet 922. The mercury formed by the decomposition is recovered via mercury outlet 923.

The overall reaction in decomposer is Sodium hydroxide and hydrogen .are end products. Mercury released in Reactions IX and XIII is returned to the electrolyzer.

The hereinabove described process steps have been coordinated into the combined process of the present invention. Intermediate and end products are utilized to produce the required chemicals, thus minimizing raw material requirement. The only by-product from the system is hydrogen, which may be discharged into the atmosphere or used as fuel or for chemical manufacture.

The main advantages to the pulp mill of the process according to the present invention are:

(a) The pulp mill may produce its own basic chemicals and thus better its own competitive position in the manufacture of bleached pulp.

(b) The process of the present invention includes a feature for balancing the output with the pulp mills chemical requirement. It is generally known that chlorine dioxide possesses properties which compared to chlorine yields an overall better quality bleached pulp. Chlorine dioxide is produced at a cost which approaches that of chlorine, comparing bleaching equivalents, and it is thus feasible to replace a large part of the chlorine, in the bleaching, with chlorine dioxide. The system utilizes this to balance chemical output with requirement.

(c) Higher than normal quantities of chlorine dioxide are used in the bleaching and a proportionally lower quantity of chlorine.

(d) The sodium sulfide produced is added directly to the green or white liquor streams in place of sodium sulfate (salt cake) to the black liquor system. The Kcal normally consumed in reducing sodium sulfate are therefore available to produce additional steam. For mills where the recovery boiler is a bottleneck the reduction in solids fired may be of major importance.

Hydrogen is produced in the chlorate cells and in the amalgam decomposer. Part of the hydrogen is used by the combustion with C1 to produce hydrogen chloride. The excess hydrogen 2.1 kg./ton is available for fuel.

The process of the present invention shows a saving of $5.39 per ton of pulp. In addition 43% more chlorine dioxide has been used than in the more conventional bleaching method. The increase use of chlorine dioxide will yield a stronger pulp with higher viscosity, reduce brightness reversion and give some increase in pulp yield.

I claim:

1. A process for the preparation of chlorine dioxide, chlorine, hydrogen, an hydroxide selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides and a sulphide selected from the group consisting of alkali metal sulphides and alkaline earth metal sulphides for use in kraft pulp mill operations from a chloride selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, sulphur and water, which process comprises the following interrelated combination of steps:

(A) subjecting a portion of an alkali metal chloride or an alkaline earth metal chloride to a decomposition reaction, thereby to form free chlorine gas;

(B) subjecting a further portion of said alkali metal chloride or said alkaline earth metal chloride to a decomposition reaction to form the corresponding alkali metal chlorate or alkaline earth metal chlorate, and free hydrogen gas;

(C) subjecting a portion of the free chlorine gas from step (A) to reaction with a portion of the free hydrogen gas from step (B) to form hydrogen chloride (D) subjecting the hydrogen chloride gas from step (C) and a portion of the alkali metal chlorate or the alkaline earth metal chlorate from step (B) to reaction to form an alkali metal chloride or an alkaline earth metal chloride, and chloric acid;

(E) subjecting the chloric acid from step (D) and a portion of the alkali metal chlorate or alkaline earth metal chlorate of step (B) to reaction to form chlorine dioxide and chlorine gas; I

(F) subjecting sulphur to reaction with an alkali metal sulphide or an alkaline earth metal sulphide to form an alkali metal polysulphide or an alkaline earth metal polysulphide;

(G) subjecting the alkali metal polysulphide or the alkaline earth metal polysulphide of step (F) to reaction to form an alkali metal sulphide or an alkaline earth metal sulphide in sufficient quantities to provide said alkali metal sulphide or said alkaline earth metal sulphide for reaction with sulphur in step (F) and for use in the kraft pulp mill operation; and

(H) decomposing an amalgam of an alkali metal or alkaline earth metal with mercury to form an alkali metal hydroxide or an alkaline earth metal hydroxide, hydrogen gas and mercury;

wherein, the amounts of said chlorine dioxide, chlorine gas, alkali metal hydroxide or alkaline earth metal hydroxide and alkali metal sulphide or alkaline earth metal sulphide are sufficient for both maintaining said interrelated combination of steps self-sufiicient and for providing such chemicals for use in the kraft pulp mill operations.

2. The process of claim 1 wherein step (A) involves a mercury cell electrolysis procedure.

3. The process of claim 2 wherein the amalgam of the alkali metal or alkaline earth metal with mercury is fed as one of the reactants for step (G) and for step (H).

4. The process of claim 3 wherein the mercury formed in steps (G) and (H) is fed as feed for step (B).

5. The process of claim 1 wherein step (B) involves an electrolysis procedure.

6. The process of claim 1 wherein the alkali metal chloride or alkaline earth metal chloride formed in step (D) is recycled to the feed for steps'(A) or (C).

7. The process of claim 1 wherein the chlorine gas produced in step (E) is recycled as feed for step (C).

8. The process of claim 1 wherein step (E) also produces excess alkali metal ions or alkaline earth metal ions which are recycled to the feed for steps (A) or (C).

9. The process of claim 1 wherein step (A) comprises: (1) dissolving the chloride selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides in the water to form a chloride brine selected from the group consisting of alkali metal (9) dissolving the sodium chloride in the water to form sodium chloride brine;

(10) electrolyzing a portion of said brine in a mercury cell whereby to produce gaseous chlorine at the anode, a portion of which is recovered and sodium chlorides and alkaline earth metal chlorides; and amalgam at the cathode.

(2) electrolyzing a portion of said brine in a mercury 15. The process of claim 14 wherein step (G) comcell whereby to produce gaseous chlorine at the prises: anode, a portion of which is recovered and amalgam (ll) reacting a portion of the sodium amalgam from selected from the group consisting of alkali metal (10) above with sodium polysulphide whereby to amalgams and alkaline earth metal amalgams at the form mercury for (10) above and to form sodium cathode. sulphide, a portion of which is recovered;

10. The process of claim 9 wherein step (G) com- (12) reacting a portion of said sodium sulphide from prises: (11) above with sulphur, whereby to form sodium (3) reacting a portion of the amalgam selected from polysulphide for (11) above.

the group consisting of alkali metal amalgams and 16. The process of claim 14 wherein step (H) comalkaline earth metal amalgams from (2) above with prises:

a polysulphide selected from the group consisting of (13) decomposing in an electrolytic decomposer a poralkali metal polysulphides and alkaline earth metal tion of sodium amalgam from (10) above, whereby polysulphides whereby to form mercury for (2) 0 to form sodium hydroxide solution, a portion of above and to form a sulphide selected from the which is recovered; gaseous hydrogen, a portion of group consisting of alkali metal sulphides and alkawhich is recovered; and mercury for (10) above. line earth metal sulphides, a portion of which is 17. The process of claim 15 wherein step (H) comrecovered; and prises:

(4) reacting a portion of said sulphide selected from (14) decomposing in an electrolytic decomposer a porthe group consisting of alkali metal sulphides and tion of sodium amalgam from (10) above, whereby alkaline earth metal sulphides from (1) above with to form sodium hydroxide solution, a portion of sulphur, whereby to form a polysulphide selected which is recovered; gaseous hydrogen, a portion of from the group consisting of alkali metal polywhich is recovered; and mercury for (10) above. sulphides and alkaline earth metal polysulphides for 18. The process of claim 9 wherein step (B) com- (1) above. prises:

11. The process of claim 9 wherein step (H) com- (15) electrolyzing a mixture of a portion of the brine prises: of (1) above with brine from (17) below, whereby (5) decomposing a portion of an amalgam selected to form sodium chlorate, and to form gaseous hydrofrom the group consisting of alkali metal amalgams 5 gen, 2. portion of which is recovered; and alkaline earth metal amalgams from (2) above (16) reacting a portion of said gaseous chlorine from whereby to form an hydroxide solution selected from (2) above with a portion of said gaseous hydrogen the group consisting of alkali metal hydroxides and from (15) above to form hydrogen chloride in gasealkaline earth metal hydroxides, a portion of which ous or aqueous solution form; is recovered; gaseous hydrogen, a portion of which (17) reacting a portion of said sodium chlorate from is recovered; and mercury for (2) above. (15) above with a portion of said hydrogen chloride 12. The process of claim 10 wherein step (H) comfrom (16) above, whereby to form sodium chloride prises: for (15) above and for (2) above and water for (5) decomposing a portion of an amalgam selected electrolysis for (1) above, and to form and recover from the group consisting of alkali metal amalgams chlorine dioxide.

and alkaline earth metal amalgams from (2) above whereby to form an hydroxide solution selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides, a portion of which is recovered; gaseous hydrogen, a portion of which 5 is recovered; and mercury for (2) above.

13. The process of claim 9 wherein step (B) comprises:

(6) electrolyzing a mixture of brine of (1) above with 19. Apparatus for the preparation of chlorine dioxide, chlorine, hydrogen, sodium hydroxide and sodium sulphide for use in kraft pulp mill operations from sodium chloride, sulphur and waterwhich process comprises the following interconnected series of units:

(A) (i) a mercury cell operative to produce gaseous chlorine at the anode and sodium amalgam at the cathode;

(ii) means at the anodic outlet of (A) (i) for rebrine formed by (8) below, whereby to form a covering a portion of the chlorine produced at chlorate selected from the group consisting of alkali the anode;

metal chlorates and alkaline earth metal chlorates, (B) (i) an electrolytic cell operative to electrolyze sodiand to form gaseous hydrogen, a portion of which um chloride brine to form sodium chlorate and gaseis recovered; ous hydrogen;

(7) reacting a portion of said gaseous chlorine from (ii) means connected to electrolytic cell (B)(i) (2) above with a portion of said gaseous hydrogen for the recovery of a portion of the gaseous from (6) above to form hydrogen chloride in gasehydrogen so-formed; ous or aqueous solution form; (C) (i) a hydrogen chloride combustion chamber oper- (8) reacting a portion of said chlorate selected from ative to produce hydrogen chloride;

the group consisting of alkali metal chlorates and (ii) means interconnecting the inlet of said comalkaline earth metal chlorates from (6) above with bustion chamber (C) (i) with a gaseous hydroa portion of said hydrogen chloride from (7) above gen outlet from said electrolytic cell (B) (i) and whereby to form a chloride selected from the group for interconnecting the inlet of said combustion consisting of alkali metal chlorides and alkaline earth chamber (C) (i) with a gaseous chlorine outlet metal chlorides for (6) above, and to form gaseous from mercury cell (A) (i); and chlorine for (7) above and Water for electrolysis (iii) means for the recovery of a portion of said for (6) above, and to form and recover chlorine hydrogen from electrolytic cell (C) (i); dioxide. (D) (i) a chlorate electrolyser and generator operative 14. The process of claim 1 wherein step (A) comto produce sodium chlorate and hydrogen;

prises: (E) (i) a chlorine dioxide generator operative to form gaseous chlorine; Water; sodium chloride; and chlorine dioxide;

(ii) means connecting the inlet of said chlorine dioxide generator (E) (i) with the chlorate product effluent line of chlorate generator (D)(i) and means connecting the outlet of said chlorine dioxide generator (E) (i) with the sodium chlorate inlet line of electrolytic cell (B)(i);

(iii) outlet means from chlorine dioxide generator (E) (i) to recover said chlorine dioxide; and

(iv) means connecting a chlorine outlet from generator (E) (i) with the inlet to combustion chamber (C) (i);

(F) (i) a sulphide reactor operative to form mercury and sodium sulphide;

(ii) means connecting the mercury outlet line of said sulphide reactor (C) (i) with the mercury inlet line of mercury cell (A) (i) and/or to amalgam decomposer (H) (i);

(iii) means connecting the inlet of said reactor (C) (i) with the amalgam outlet line of said mercury cell (A) (i); and

(iv) means for the recovery of a portion of said sodium sulphide of (C) (i);

(G) (i) a polysulphide reactor operative to form sodium polysulphide;

(ii) inlet means for the introduction of sulphur to said reactor (G) (i);

(iii) means connecting the sodium sulphide outlet of said sulphide reactor (C) (i); with the inlet of said reactor (G) (i);

(iv) means connecting the polysulphide outlet of 18 said reactor (G) (i) with the inlet of said reactor (C) (i); and (H) (i) a decomposer to form sodium hydroxide solution; gaseous hydrogen and mercury;

(ii) means connecting the amalgam outlet of mercury cell (A) (i) with the inlet of said decomposer (H)(i) and/or means for connecting excess amalgam outlet means from sulphide reactor (F) (i);

(iii) inlet means for water to said decomposer (H) (iv) means for the recovery of product hydroxide from decomposer (H) (i);

(v) means for the recovery of product hydrogen from decomposer (H) (i); and

(vi) means connecting the mercury outlet of decomposer (H) (i) with the mercury inlet of mercury cell (A) (i).

References Cited UNITED STATES PATENTS 2,484,402 10/1949 Day et a1 23152 2,628,935 2/1953 Earnest et al. 204 3,051,637 8/1962 Judicc et a1 20498 JOHN H. MACK, Primary Examiner. D. R. JORDAN, Assistant Examiner.

US. Cl. X.R. 

1. A PROCESS FOR THE PREPARATION OF CHLORINE DIOXIDE, CHLORINE, HYDROGEN, AN HYDROXIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HYDROXIDES AND ALKALINE EARTH METAL HYDROXIDES AND A SULPHIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL SULPHIDES AND ALKALINE EARTH METAL SULPHIDES FOR USE IN KRAFT PULP MILL OPERATIONS FROM A CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CHLORIDES AND ALKALINE EARTH METAL CHLORIDES, SULPHUR AND WATER, WHICH PROCESS COMPRISES THE FOLLOWING INTERRELTATED COMBINATION OF STEPS: (A) SUBJECTING A PORTION OF AN ALKALI METAL CHLORIDE OR AN ALKALINE EARTH METAL CHLORIDE TO A DECOMPOSITION REACTION, THEREBY TO FORM FREE CHLORINE GAS; (B) SUBJECTING A FURTHER PORTION OF SAID ALKALI METAL CHLORIDE OR SAID ALKALINE EARTH METAL CHLORIDE TO A DECOMPOSITION REACTION TO FORM THE CORRESPONDING ALKALI METAL CHLORATE OR ALKALINE EARTH METAL CHLORATE, AND FREE HYDROGEN GAS; (C) SUBJECTING A PORTION OF THE FREE CHLORINE GAS FROM STEP (A) TO REACTION WITH A PORTION OF THE FREE HYDROGEN GAS FROM STEP (B) TO FORM HYDROGEN CHLORIDE GAS; (D) SUBJECTING THE HYDROGEN CHLORIDE GAS FROM STEP (C) AND A PORTION OF THE ALKALI METAL CHLORATE OR THE ALKALINE EARTH METAL CHLORATE FROM STEP (B) TO REACTION TO FORM AN ALKALI METAL CHLORIDE OR AN ALKALINE EARTH METAL CHLORIDE, AND CHLORIC ACID; (E) SUBJECTING THE CHLORIC ACID FROM STEP (D) AND A PORTION OF THE ALKALI METAL CHLORATE OR ALKALINE EARTH METAL CHLORATE OF STEP (B) TO REACTION TO FORM CHLORINE DIOXIDE AND CHLORINE GAS; (F) SUBJECTING SULPHUR TO REACTION WITH AN ALKALI METAL SULPHIDE OR AN ALKALINE EARTH METAL SULPHIDE TO FORM AN ALKALI METAL POLYSULPHIDE OR AN ALKALINE EARTH METAL POLYSULPHIDE; (G) SUBJECTING THE ALKALI METAL POLYSULPHIDE OR THE ALKALINE EARTH METAL POLYSULPHIDE OF STEP (F) TO REACTION TO FORM AN ALKALI METAL SULPHIDE OR AN ALKALINE EARTH METAL SULPHIDE IN SUFFICIENT QUANTITIES TO PROVIDE SAID ALKALI METAL SULPHIDE OR SAID ALKALINE EARTH METAL SULPHIDE FOR REACTION WITH SULPHUR IN STEP (F) AND FOR USE IN THE KRAFT PULP MILL OPERATION; AND (H) DECOMPOSING AN AMALGAM OF AN ALKALI METAL OR ALKALINE EARTH METAL WITH MERCURY TO FORM AN ALKALI METAL HYDROXIDE OR AN ALKALINE EARTH METAL HYDROXIDE, HYDROGEN GAS AND MERCURY;
 19. APPARATUS FOR THE PREPARATION OF CHLORINE DIOXIDE, CHLORINE, HYDROGEN, SODIUM HYDROXIDE AND SODIUM SULPHIDE FOR USE IN DRAFT PULP MILL OPERATION FROM SODIUM CHLORIDE, SULPHUR AND WATER WHICH PROCESS COMPRISES THE FOLLOWING INTERCONNEDTED SERIES OF UNITS: (A) (I) A MERCURY CELL OPERATIVE TO PRODUCE GASEOUS CHLORINE AT THE ANODE AND SOIDUM AMALGAM AT THE CATHODE; (II) MEANS AT THE ANODIC OUTLET OF (A) (I) FOR RECOVERING A PORTION OF THE CHLORINE PRODUCED AT THE ANODE; (B) (I) AN ELECTROLYTIC CELL OPERATIVE TO ELECTROLYZE SODIUM CHLORIDE BRINE TO FORM SODIUM CHLORATE AND GASEOUS HYDROGEN; (II) MEANS CONNECTED TO ELECTROLYTIC CELL (B) (I) FOR THE RECOVERY OF A PORTION OF THE GASEOUS HYDROGEN SO-FORMED; (C) (I) A HYDROGEN CHLORIDE COMBUSITON CHAMBER OPERATIVE TO PRODUCE HYDROGEN CHLORIDE; (II) MEANS INTERCONNECTING THE INLET OF SAID COMBUSION CHAMBER (C) (I) WITH A GASEOUS HYDROGEN OUTLET FROM SAID ELECTROLYTIC CELL (B) (I) AND FOR INTERCONNECTING THE INLET OF SAID COMBUSITON CHAMBER (C) (I) WITH A GASEOUS CHLORINE OUTLET FROM MERCURY CELL (A) (I); AND (III) MEANS FOR THE RECOVERY OF A PORTION OF SAID HYDROGEN FROM ELECTROLYTIC CELL (C) (I); (D) (I) A CHLORATE ELECTROLYSER AND GENERATOR OPERATIVE TO PRODUCE SODIUM CHLORATE AND HYDROGEN; (E) (I) A CHLORINE; WATER; SODIUM CHLORIDE; AND CHLOGASEOUS CHLORINE; WATER; SODIUM CHLORIDE; AND CHLORINE DIOXIDE; (III) MEANS CONNECTING THE INLET OF SAID CHLORINE DIOXIDE GENERATOR (E) (I) WITH THE CHLORATE PRODUCT EFFLUENT LINE OF CHLORATE GENERATOR (D) (I) AND MEANS CONNECTING THE OUTLET OF SAID CHLORINE DIOXIDE GENERATOR (E) (I) WITH THE SODIUM CHLORATE INLET LINE OF ELECTROLYTIC CELL (B) (I); (III) OUTLET MEANS FROM CHLORINE DIOXIDE GENERATOR (E) (I) TO RECOVER SAID CHLORINE DIOXIDE; AND (IV) MEANS CONNECTING A CHLORINE OUTLE FROM GENERATOR (E) (I) WITH THE INLET TO COMBUSTION CHAMBER (C) (I); (F) (I) A SULPHIDE REACTOR OPERATIVE TO FORM MERCURY AND SODIUM SULPHIDE; (II) MEANS CONNECTING THE MERCURY OUTLET LINE OF SAID SULPHIDE REACTOR (C) (I) WITH THE MERCURY INLET LINE OF MERCURY CELL (A) (I) AND/OR TO AMALGRAM DECOMPOSER (H) (I); (III) MEANS CONNECTING THE INLET OF SAID REACTOR (C) (I) WITH THE AMALGAM OUTLET LINE OF SAID MERCURY CELL (A) (I); AND (IV) MEANS FOR THE RECOVERY OF A PORTION OF SAID SODIUM SULPHIDE OF (C) (I); (G) (I) A POLYSULPHIDE REACTOR OPERATIVE TO FORM SODIUM POLYSULPHIDE; (II) INLET MEANS FOR THE INTRODUCTION OF SULPHUR TO SAID REACTOR (G) (I); (III) MEANS CONNECTING THE SOIDUM SULPHIDE OUTLET OF SAID SULPHIDE REACTOR (C) (I); WITH THE INLET OF SAID REACTOR (G) (I); (IV) MEANS CONNECTING THE POLYSULPHIDE OUTLET OF SAID REACTOR (G) (I) WITH THE INLET OF SAID REACTOR (C) (I); AND (H) (I) A DECOMPOSER TO FORM SODIUM HYDROXIDE SOLUTION; GASEOUS HYDROGEN AND MERUCRY; (II) MEANS CONNECTING THE AMALGRAM OUTLET OF MERCURY CELL (A) (I) WITH THE INLET OF SAID DECOMPOSER (H) (I) AND/OR MEANS FOR CONNECTING EXCESS AMALGAM OUTLET MEANS FROM SULPHIDE REACTOR (F) (I); (III) INLET MEANS FOR WATER TO SAID DECOMPOSER (H) (I); (IV) MEANS FOR THE RECOVERY OF PRODUCT HYDROXIDE FROM DECOMPOSER (H) (I); (V) MEANS FOR THE RECOVERY OF PRODUCT HYDROGEN FROM DECOMPOSER (H) (I); AND (VI) MEANS CONNECTING THE MERCURY OUTLET OF DECOMPOSER (H) (I) WITH THE MERCURY INLET OF MERCURY CELL (A) (I). 